The present invention relates to a fuel injector for a combustion engine and to pressure control of the opening an closing of an injector valve.
One way of reducing discharges of emissions from diesel engines is to inject the fuel at a very high pressure. A so-called “Common Rail system” is commonly used for effecting injection at a high pressure in the combustion spaces of a diesel engine. A Common Rail system comprises a high-pressure pump which pumps fuel at a high pressure to an accumulator tank (“Common Rail”). The pressure in the accumulator tank during operation may be within the range 250 to 1600 bar. The fuel in the accumulator tank is intended to be distributed to all the cylinders of the combustion engine. Fuel from the accumulator tank is injected into the combustion spaces of the respective cylinders by electronically controlled injection means. The injection means comprise injection valves which have to be able to open and close very quickly. The injection valves are controlled by an electrical control unit which substantially continuously calculates the amount of fuel to be supplied to the respective cylinders on the basis of information about various engine parameters, e.g. the engine's load and speed.
Another known practice is to provide injection means with a built-in hydraulic servo unit for imparting a further pressure rise to the fuel delivered from an accumulator tank (Common Rail) before it is injected into the combustion engine. Such a very high injection pressure imposes very high strength requirements for the components of the injection means. To enable the high injection pressure to be maintained for the whole injection period, the injection valves need to be able to open and close very quickly. The injection valves are usually so designed that they are moved to an open position by the pressurised fuel against the action of a spring means, and to a closed position by the spring means when the pressure of the pressurised fuel has decreased. To enable the closing process to take place very quickly when the high fuel pressure decreases, a very powerful spring means has to be used. Such spring means may have to be specially made in order to provide the necessary force and have a service life similar to surrounding components which are subject to the high fuel pressure. Injection means for injecting fuel at a very high pressure in a combustion engine are therefore relatively expensive to manufacture.